1. Field of the Invention
This invention relates to data processing printers, and more particularly to a ribbon shield for a dot-matrix printer ribbon guide.
2. Description of Related Art
FIG. 1 discloses a perspective view of a dot-matrix type data processing printer printhead mechanism 10 that would be mounted in a printer such that it may traverse the width of a print medium such as paper (not shown). An inked ribbon 12 is threaded through a ribbon guide 13 such that the ribbon 12 is interposed between wire printer impact elements (not shown) of the printhead mechanism 10 and the paper.
FIG. 2 is a closeup perspective view of a typical prior art ribbon guide 13 (for example, the illustrated ribbon guide may be used in a Centronics 350 printer, manufactured by the Centronics Data Computer Corporation of Hudson, N.H.). Shown is an attachment fixture 20 for affixing the ribbon guide 13 to the printhead mechanism 10. A short guide wing 21 and a long guide wing 22 guide the ribbon 12 away from the paper from a ribbon supply area to a ribbon take-up area (not shown). A slot 23 running the width of the front face of the ribbon guide 13 permits insertion of the ribbon 12 into the guide.
The ribbon guide 13 maintains a section of the ribbon 12 juxtaposed against the paper. The ribbon 12 spans an opening 24 in the ribbon guide 13 from a first edge 25 to a second edge 26. The exposed section of the ribbon 12 (approximately 1/2".times.1/2" in size), if unshielded, potentially would smear the paper with ink as the printhead mechanism 10 traverses the width of the paper. Therefore, in the existing art, a "ribbon shield" has been attached to the ribbon guide 13 to cover the majority of the ribbon section exposed by the ribbon guide 13, leaving only a small opening through which the printer impact elements may press the ribbon 12 against the paper to produce desired impressions.
Ribbon shields in the past have proven inadequate or disadvantageous in a number of ways. One such ribbon shield 30, shown in FIG. 2, is made of plastic typically having a thickness of approximately 0.0075 inches. It has a long wing 31 and a short wing 32 that are attached to the outer face of a ribbon guide 13 of the type shown in FIG. 2 by means of small hook-like projections (not shown) on the ribbon guide 13 that engage small holes 33, 34 in the plastic ribbon shield 30. A small printer impact element opening 35 is provided to permit the printer impact elements to press the ribbon 12 against the paper.
There are several disadvantages to this prior art plastic ribbon shield 30. It is difficult to assemble properly to the front face of the ribbon guide 13, and thus relatively expensive to manufacture. As its surface is dragged across the paper when the printhead mechanism 10 traverses the width of the paper, the plastic is subjected to wear and frequently breaks, falling off entirely or jamming in the printer mechanism. Another disadvantage of this type of ribbon shield is that it requires a relatively large gap between the printer impact elements and the paper, which limits print quality. In one printer in which such a plastic, front-surface mounted print shield 30 is used, the printhead mechanism must be placed in its No. 4 print position, rather than a position closer to the paper, in order to minimize jamming of the printer mechanism.
FIG. 3 shows another prior art ribbon shield 40 that has several advantages over the plastic ribbon shield 30 described above, yet does not completely eliminate certain problems. This ribbon shield 40 is manufactured typically of stainless steel having a thickness of 0.004 inches. The ribbon shield 40 has a shield face 43 and two short (typically 1/16" to 2/16") insert tabs 41, 42 on either side of the shield face 43. The ribbon shield 40 is attached to the foremost surface of a ribbon guide 13 of the type shown in FIG. 3 by inserting the tabs 41, 42 under the corresponding edges 25, 26 of the ribbon guide 13. This metal ribbon shield 40 allows for closer printhead mechanism 10 settings than the plastic version 30 (typically at the No. 2 position rather than the No. 4 position in the same printer), and does not generally wear out or fall off. However, because it is relatively thick and stiff, and must be bent somewhat in the middle of the shield face (along a line A dividing the print shield 40 about in half longitudinally), it is difficult to factory install without "kinking" (i.e., applying stress beyond its yield point) the shield. This is particularly a problem since the shield face is weakened by a printer impact element opening 44. An end-user would have a difficult time removing this ribbon shield 40 should it be desired to set the printer to its closest printhead setting, and it would be even more difficult for an end-user to reinstall this type of ribbon shield without permanently kinking the part.
Therefore, it is desirable to have a printer ribbon shield that is easily installed, easily removable, durable, unlikely to kink during installation or removal, and which permits printhead gap settings from the paper of the minimum distance capable in a printer.